What would you change in Heroes of Might and Magic IV?

In the fourth installment, I would personally add the Ressurection spell.

Resurrection? It's already there :) in 3.51

In my opinion, the magic system in the fourth game is not well-developed at all.

What don't you like about it?:) In fact, all the magic is very well distributed among the schools, and all the schools are uniquely different.

As for mass slow, haste, and bless, it's not all connected:p

There seem to be fewer field spells, but on the other hand, the exclusion of the cheesy (in my opinion) teleportation spell from the third game, as well as the door and flight spells, is a big plus. And there are some quite interesting spells like "quicksand" and "endurance."
True, in the fourth game, teleportation was also cheesy for a while, because it worked in battles :)